I was going thru all of the 183 pages of the mods on this forum on TT. I saw someone ran a seperate electric cord from the outside to plug in their portable heater.

My question is the wiring in a TT made strong enough to run a electric heater. The type I use is a small one just to take the chill out of the camper at night. It maybe 12 inches high by 12 inches wide with a built in fan.

Your TT wiring can probably handle a total of 30 amps being drawn with individual circuits being rated at 15A. This is OK to run an electric heater up to 1500watts.
Using a separate power feed for the heater basically frees up 12amps or so from the main circuit so you can run other high draw stuff (microwave, water heater, kettle, hair dryer etc) at the same time without tripping the main breaker.
Not much point if nothing else is being used.

Reason some run an extension cord from 20A receptacle to operate electric heater is because they are overloading the 15A breaker in their trailer.........too many devices on same circuit. Same thing happens in a S&B.

We use an electric heater to take chill out of living room every time it gets cold.

Yes, your TT wiring should be able to handle the load of a small electric heater. Models of electric heaters that are rated for 1500 watts of continuse output would draw about 12 amps. This is under the rating of 15 amps, the lowest rating of any circuit in the TT. If you choose to add other devices to the same circuit, i.e. lights, vacuum cleaner, fans, etc. you could run into problems in very short order.
If your TT has a 20 amp circuit (this can be found by looking in the AC panel board), it would be good to use that branch circuit to run the heater.

Check the amp rating for the breakers in your electrical supply center. You will probably find that you have 20 amp breakers or 15 amp breakers on EACH circuit. A 120 volt electric heater is not ALLOWED to draw more than 1500 watts maximum power so either of those circuits would handle an electric heater if nothing else were running on the same circuit. The power coming INTO the trialer will be maxed out a 30 amps total for all circuits combined. If nothing else is running on electricity and you plug them into different circuits in the trailer you could run two at once on full power. Not OFTEN though that you don't have other things drawing current in your RV.

Just make sure your electric heater has a tip over switch so if it tips over it shuts off. I believe all of them should have this by law but, some don't. For the ones that don't if it get accidentally knocked over it keeps running and can set the floor on fire. If it's setting on carpet this happens purty quickly.

I am one of the ones who runs in a separate 12 awg cord to run a separate 1500 watt heater.

I do it because some times we use 2 heaters. An oil filled one and a fan forced ceramic. We have a big camper with a lot of windows.

It depends on how your camper is wired. In mine there is a general purpose 15 amp circuit that feeds the wall plugs in the living area, the bath and the bed room. The key to understanding is to only use 1 heat producing device on that line at once. Having the heater on in the living area and someone using a hair dryer in the bath will be a problem if yours is wired this way.

Mine also has a more dedicated 15 amp receptacle in the galley area. The fridge is tied to this one as well but nothing else. There is more power available there. In our case the TT manufacture knew people at times plug in coffee pots, electric fry pans etc in the galley plug. If you use that plug then your general purpose plugs can run other things.

Just remember, when running a 1500 watt heater be noted that it is a large draw. Using the HW heater (1400 watts) on electric, the fridge on electric (350 watts), using a lot of DC for lights on standard 921 bulbs and the battery run down some makes the converter pull power too (600 watts). Now fire up the microwave on hi (1500 watts) can break the bank. Total 5,350 watts/120 volts = 44.6 amps which is over the 30 amp main.

Use the HW heater on gas and or shut off the electric heater when running the microwave. It is all about balancing the loads. The wiring should be sized and have the right breaker to protect it. Just know the limits and stay within them.